1,2,4-triazolo[4,3-B]pyrido[3,2-D]pyridazine derivatives and pharmaceutical compositions containing them

ABSTRACT

Heterocyclic compounds of formula (I),  
                 
 
     wherein R 1  represents a hydrogen atom or a —(CH 2 ) M —Y group, wherein m is an integer from 0 to 4 and Y represents an alkyl, haloalkyl, alkoxy, alkoxycarbonyl, C 3 -C 7  cycloalkyl, norbornyl or phenylalkenyl group, or an aromatic group which aromatic group ^ may optionally be substituted by one or more halogen atoms; R 2  represents an aromatic group which aromatic group may optionally be substituted by one or more halogen atoms or alkyl, alkoxy, C 3 -C 6  cycloalkoxy, methylenedioxy, nitro, dialkylamino or trifluoromethyl groups, and R 3  represents a hydrogen or halogen atom or an alkyl group, and pharmaceutically acceptable salts thereof, processes for preparing the same. The compounds are phosphodiesterase 4 inhibitors.

[0001] This invention relates to new therapeutically useful heterocyclic compounds, to process for their preparation and to pharmaceutical compositions containing them.

[0002] It is known that inhibitors of phosphodiesterase 4 (PDE 4) are useful in the treatment of inflammatory and allergic processes such as asthma, non-steroidal antiinflammatory drugs-induced gastrointestinal damage and atopic dermatitis.

[0003] EP-A-85,840 discloses a series of triazolo-phthalazine derivatives of formula:

[0004] which are useful as anxiolytic agents.

[0005] We have now found that the presence of a pyridine ring instead of the benzo ring in the above structure, provides new compounds which inhibit cyclic phosphodiesterases, in particular type 4 cyclic phosphodiesterases and have a very low emetic activity (10-100 times less active than rolipram in inducing emesis in dogs).

[0006] Accordingly, the present invention provides a compound which is a heterocycle of formula (I):

[0007] wherein:

[0008] R¹ represents a hydrogen atom or a —(CH₂)_(m)—Y group, wherein m is an integer from 0 to 4 and Y represents an alkyl, haloalkyl (preferably trifluoromethyl), alkoxy, alkoxycarbonyl, C₃-C₇ cycloalkyl, norbornyl (preferably 2-norbornyl) or phenylalkenyl group, or an aromatic group (preferably phenyl or pyridyl) which aromatic group Y may optionally be substituted by one or more halogen atoms;

[0009] R² represents an aromatic group (preferably phenyl, naphthyl or thienyl) which aromatic group may optionally be substituted by one or more halogen atoms or alkyl, alkoxy, C₃-C₆ cycloalkoxy, methylenedioxy, nitro, dialkylamino or trifluoromethyl groups; and

[0010] R³ represents a hydrogen or halogen atom (preferably chloro) or an alkyl group,

[0011] and pharmaceutically acceptable salts thereof.

[0012] The alkyl, haloalkyl, alkenyl or alkynyl groups and moieties, such as in the alkoxy groups, mentioned in relation to the groups R¹—R³ in compounds of the invention are usually “lower” alkyl, that is containing up to 6 and particularly up to 4 carbon atoms, the hydrocarbon chain being branched or straight. Examples of alkyl groups and moieties are CH₃, C₂H₅, C₃H₇, i-C₃H₇, n—C₄H₉, i—C₄H₉, isoamyl and neopentyl.

[0013] When any of the groups, such as R¹ or R² has a chiral centre, the compounds of formula (I) exhibit optical isomerism and the isomers are within the scope of the present invention.

[0014] Examples of R¹ are the preferred alkyl groups mentioned above, cyclopropyl, cyclopropylmethyl, cyclobutyl, cyclobutylmethyl, cyclopentyl and cyclopenthylmethyl.

[0015] Examples of R² are phenyl, 3-chlorophenyl, 4-chlorophenyl, 3-fluorophenyl, 4-fluorophenyl and 3-nitrophenyl.

[0016] Examples of R³ are hydrogen, alkyl or chloro, preferably in the 8- or 9-positions.

[0017] The most preferred compounds of the invention are

[0018] 6-(4-fluorophenyl)-3-isobutyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine, 3-cyclopropylmethyl-6-(3-nitrophenyl)-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine, 3-cyclopropyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine, and 3-cyclobutylmethyl-6-(3-nitrophenyl)-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine.

[0019] According to a further feature of the present invention, the heterocyclic compounds of formula (I) can be prepared from the corresponding hydrazine derivative of formula (II):

[0020] wherein

[0021] R² and R³ are as defined above, by reaction with a reactive derivative of a carboxylic acid of the general formula (III):

HOOC—R¹  (III)

[0022] wherein R¹ is as defined above. The reactive derivative of the said carboxylic acid may be, for example, a halide (preferably chloride), an anhydride or a mixed anhydride.

[0023] The reaction is preferably carried out in an inert organic solvent such as methylene chloride, dioxane or tetrahydrofuran, in the presence of an organic nitrogen-containing base, e.g. triethylamine and at a temperature between −10° C. and +60° C. In the reaction, the corresponding hydrazide of general formula (IV) is first formed:

[0024] wherein R¹, R² and R³ are as defined above. A suspension of this hydrazide (IV) in an organic solvent such as dioxane, tetrahydrofuran, isopropanol or n-butanol, is heated, for example at the boiling point of the solvent, to give the corresponding heterocyclic compound of formula (I).

[0025] The hydrazine derivative of formula (II) may be prepared by:

[0026] 1) reacting a hydrazone of formula (V):

[0027] wherein R² and R³ are as defined above and R⁴ is an alkyl group, with a phosphorus halide or phosphorus oxyhalide (preferably phosphorus oxychloride), to form the intermediate compound of formula (VI):

[0028] wherein R² and R³ are as defined above and X is a chlorine or bromine atom;

[0029] 2) reacting compound (VI) with an alkyl carbazate (preferably t-butyl carbazate) of formula (VII):

H₂N—NH—COOR⁵  (VII)

[0030] wherein R⁵ is an alkyl group, to give the alkoxycarbonylhydrazine derivative (VIII):

[0031] wherein R², R³ and R⁵ are as defined above; and

[0032] 3) treating compound (VIII) with hydrogen chloride in an anhydrous solvent as ethanol.

[0033] The reaction between the hydrazone of formula (V) and a phosphorus halide or phosphorus oxyhalide is carried out with an excess of reagent at a temperature from 80° C. to 120° C., then removed the excess of reagent and poured into cold water. In this way the compound (VI) is obtained.

[0034] The reaction of (VI) with the alkyl carbazate of formula (VII) to obtain the corresponding alkoxycarbonylhydrazine derivative (VIII), is preferably carried out in the presence of an organic solvent as tetrahydrofuran or dioxan at a temperature of from 60° C. to the boiling point of the reaction medium.

[0035] The alkoxycarbonylhydrazine derivative (VIII) may, for example, be transformed into the hydrazine derivative (II) at room temperature in hydrogen chloride-ethanol saturated solution.

[0036] The hydrazone derivatives of formula (V) are known compounds which can be prepared from the corresponding 2-acylnicotinic acid by known methods described in the literature.

[0037] The inhibition of cyclic nucleotide phosphodiesterase 4 from guinea-pig hearts was performed using 96-well microtiter plates as described by Verghese et al., (Molecular Pharmacology, 47, 1164-1171 (1995)).

[0038] The results from such test are shown in Table 1. TABLE 1 PDE4 Compound * IC₅₀ (μM) A 10 6 2 7 0.3 12 3 31 0.2 47 0.7 55 0.2 60 0.1 61 2 109 0.04 112 0.7 113 0.2

[0039] Compound A is 3-isobutyl-6-phenyl-1,2,4-triazolo[3,4-a]phthalazine, a compound included in EP-A-8-5,840.

[0040] As it can be seen from Table 1, the compounds of formula (I) are cyclic phosphodiestrerase inhibitors, in particular type 4 cyclic AMP phosphodiesterase inhibitors. The compounds are also capable of blocking the production of some pro-inflammatory cytokines such as, for example, TNFα. Thus, they can be used in the treatment of allergic, inflammatory and immunological diseases, as well as those disease or conditions where the blockade of pro-inflammatory cytokines or the selective inhibition of PDE 4 could be of benefit.

[0041] These diseases states include asthma, rheumatoid arthritis, osteoarthritis, osteoporosis, bone-formation disorders, glomerulonephritis, multiple sclerosis, Graves ophtalmopathy, myasthenia gravis, insulin-dependent diabetes mellitus, graft rejection, gastrointestinal disorders such as ulcerative colitis or Crohn disease, septic shock, adult distress respiratory syndrome, and skin diseases such as atopic dermatitis, contact dermatitis, acute dermatomyositis and psoriasis.

[0042] They can also be used as improvers of cerebrovascular function as well as in the treatment of other CNS related diseases such as dementia, Alzheimer's disease, depression, and as nootropic agents.

[0043] The compounds of the present invention are also of benefit when administered in combination with other drugs such as steroids and immunosuppressive agents, such as cyclosporin A, rapamycin or T-cell receptor blockers. In this case the administration of the compounds allows a reduction of the dosage of the other drugs, thus preventing the appearance of the undesired side effects associated with both steroids and immunosuppressants.

[0044] The compounds of the invention have also shown their efficacy in blocking, after preventive and/or curative treatment, the erosive and ulcerogenic effects induced by a variety of etiological agents, such as antiinflammatory drugs (steroidal or non-steroidal antiinflammatory agents), stress, ammonia, ethanol and concentrated acids. They can be used alone or in combination with antacids and/or antisecretory drugs in the preventive and/or curative treatment of gastrointestinal pathologies like drug-induced ulcers, peptic ulcers, H. Pylori-related ulcers, esophagitis and gastro-esophageal reflux disease.

[0045] They can also be used in the treatment of pathological situations where damage to the cells or tissues is produced through conditions like anoxia or the production of an excess of free radicals. Examples of such beneficial effects are the protection of cardiac tissue after coronary artery occlusion or the prolongation of cell and tissue viability when the compounds of the invention are added to preserving solutions intended for storage of transplant organs or fluids such as blood or sperm. They are also of benefit on tissue repair and wound healing.

[0046] The present invention also provides a heterocyclic compound of formula (I) for use in a method of treatment of the human or animal body by therapy, particularly for use as a PDE 4 inhibitor or to block the production of a pro-inflammatory cytokine such as TNFα.

[0047] The present invention additionally provides a pharmaceutical composition which comprises, as active ingredient, at least one heterocyclic compound of formula (I), and a pharmaceutically acceptable carrier or diluent.

[0048] Preferably the compositions are in a form suitable for oral, inhalation, rectal, transdermal, nasal, topical or parenteral administration.

[0049] The pharmaceutically-acceptable carriers or diluents which are admixed with the active compound or compounds to form the compositions of this invention are well known per se and the actual excipients used depend inter alia on the intended method of administration of the compositions.

[0050] Compositions of this invention are preferably adapted for administration per os. The compositions for oral administration may take the form of tablets, capsules, lozenges or effervescent granules or liquid preparations such as elixirs, syrups or suspensions, all containing one or more compounds of the invention. Such preparations may be made by methods well known in the art, for instance by mixing the heterocyclic compound of formula (I) with the pharmaceutically acceptable carrier or diluent.

[0051] The diluents which may be used in the preparation of the compositions include those liquid and solid diluents which are compatible with the active ingredient, together with colouring or flavouring agents if desided. Tablets or capsules may conveniently contain from 1 to 100 mg and preferably from 5 to 50 mg of active ingredient. The compounds may also be incorporated into pellets coated with appropriate natural or synthetic polymers known in the art to produce sustained release characteristics or incorporated with polymers into tablet form to produce the same characteristics.

[0052] The liquid compositions adapted for oral use may be in the form of solutions, suspensions or aerosols. The solutions may be aqueous or aqueous-alcoholic solutions in association with, for example, sucrose or sorbitol to form a syrup. The suspensions may comprise an insoluble or microencapsulated form of an active compound of the invention in association with water and other acceptable solvents together with a suspending agent or flavouring agent.

[0053] Compositions for inhalation administration may be in the form of solutions, suspensions or micronized powder, contained in an appropriate inhaler.

[0054] Compositions for parenteral injection may be prepared, which may or may not be freeze-dried and which may be dissolved in water or an appropriate parenteral injection fluid.

[0055] In human therapy, the doses of the heterocyclic compound depend on the desired effect and duration of the treatment; adult doses are generally from 1 mg to 100 mg per day. In general the physician will decide the posology, taking into account the age and weight of the patient being treated.

[0056] The following Examples further illustrate the invention.

EXAMPLE 1

[0057] a) A mixture of t-butoxycarbonylhydrazone of 2-benzoylnicotinic acid (45 g; 13.2 mols) in phosphorus oxychloride (500 ml) was boiled under reflux for one hour, then the excess of phosphorus oxychloride was removed under reduced pressure, the residue treated with ice-water and extracted twice with methylene chloride. The organic solution was washed with 4% sodium bicarbonate aqueous solution, with brine and after drying (Na₂SO₄), the solvent removed in vacuo. The obtained solid was collected with a mixture of diethyl ether-petrol ether 1:1 to give 5-chloro-8-phenylpyrido[2,3-d]pyridazine as a red solid, (25.4 g; 80% yield).

[0058] b) To a suspension of the above compound (18.2; 0.075 mols) in anhydrous tetrahydrofuran (180 ml), t-butyl carbazate (10.0 g; 0.075 mols) was added and the mixture was boiled under reflux for one hour. After cooling the crystallized solid was collected by filtration when 5-t-butoxycarbonylhydrazino-8-phenylpyrido[2,3-d]pyridazine was obtained (28.5 g). This compound was solved in ethanol (150 ml), hydrogen chloride in ethanol saturated solution (100 ml) was added and the resulting mixture stirred at room temperature for 15 hours. A solid was formed which was collected by filtration and washed with diethyl ether to give 5-hydrazino-8-phenylpyrido[2,3-d]pyridazine dihydrochloride (21.6 g; 92% yield).

[0059] c) To a suspension of 5-hydrazino-8-phenylpyrido[2,3-d]pyridazine dihydrochloride (1.24 g; 0.004 mols) in methylene chloride (30 ml), triethylamine (1.9 ml; 0.013 mols) was added, then stirred at room temperature for 15 minutes and pivaloyl chloride (0.5 ml; 0.0044 moles) slowly added. After stirring at room temperature for two hours, water (30 ml) was added, the formed yellow solid, collected by filtration and washed with diethyl ether to give the intermediate hydrazide. This compound was suspended in n-butanol (30 ml), boiled under reflux for 15 hours and on cooling, crystallized a white solid which was collected by filtration and washed with diethyl ether. The obtained solid was purified by flash column chromatography with silica gel and methylene chloride-ethanol-ammonium hydroxide 200:8:1 as eluent. 3-t-butyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine was obtained (0.83 g; 69% yield), m.p. 188.1 (determined by Differential Scanning Calorimetry, Perkin-Elmer DSC-7 (compound 8 in Table 2).

[0060] The heterocyclic compounds of formula (I) in Table 2 were prepared according to the processes disclosed in this Example, but with the appropriate starting materials. TABLE 2

Compound R¹ R² R³ m.p. ° C. 1 H C₆H₅ H 215.8 2 CH₃ ″ ″ 215.9 3 C₂H₅ ″ ″ 194.1 4 C₃H₇ ″ ″ 168.1 5 i-C₃H₇ ″ ″ 176.8 6 n-C₄H₉ ″ ″ 162.9 7 i-C₄H₉ ″ ″ 179.7 8 t-C₄H₉ ″ ″ 188.1 9 n-C₅H₁₁ ″ ″ 137.4 10 neopentyl ″ ″ 216.3 11 t-amyl ″ ″ 153 12 cyclopropyl ″ ″ 244.3 13 cyclobutyl ″ ″ 218 14 cyclopentyl ″ ″ 202.4 15 cyclohexyl ″ ″ 196.3 16 cyclopropyl-CH₂ ″ ″ 195 17 cyclobutyl-CH₂ ″ ″ 183 18 cyclopentyl-CH₂ ″ ″ 193 19 cyclohexyl-CH₂ ″ ″ 212.8 20 2-norbornyl-CH₂ ″ ″ 217 21 C₆H₅ ″ ″ 304.1 22 C₆H₅—CH₂ ″ ″ 192 23 C₆H₅—CH₂CH₂ ″ ″ 176 24 C₆H₅—CH═CH ″ ″ 278 25 CF₃ ″ ″ 192.5 26 H₃CO—CH₂ ″ ″ 159 27 2-ClC₆H₄ ″ ″ 206 28 4-pyridyl ″ ″ 333.4 29 CH₃ 4-FC₆H₄ ″ 276 30 n-C₄H₉ ″ ″ 111 31 i-C₄H₉ ″ ″ 135 32 t-C₄H₉ ″ ″ 195 33 neopentyl ″ ″ 216 34 cyclopropyl ″ ″ 245 35 cyclohexyl ″ ″ 177 36 cyclopropyl-CH₂ ″ ″ 160 37 cyclobutyl-CH₂ ″ ″ 132 38 cyclopentyl-CH₂ ″ ″ 162 39 2-norbornyl-CH₂ ″ ″ 161 40 C₆H₅—CH═CH ″ ″ 272 41 C₂H₅OOC—CH₂ ″ ″ 42 i-C₄H₉ 3-FC₆H₄ ″ 147 43 neopentyl ″ ″ 190 44 cyclopropyl ″ ″ 222 45 cyclopropyl-CH₂ ″ ″ 174 46 cyclobutyl-CH₂ ″ ″ 139 47 cyclopentyl-CH₂ ″ ″ 145 48 i-C₄H₉ 2-FC₆H₄ ″ 202 49 t-C₄H₉ ″ ″ 212 50 neopentyl ″ ″ 235 51 cyclopropyl ″ ″ 262 52 cyclopropyl-CH₂ ″ ″ 224 53 i-C₄H₉ 4-ClC₆H₄ ″ 133 54 cyclopropyl ″ ″ 208 55 i-C₄H₉ 3-ClC₆H₄ ″ 113 56 t-C₄H₉ ″ ″ 160 57 neopentyl ″ ″ 177 58 t-amyl ″ ″ 150 59 cyclopropyl ″ ″ 189 60 cyclopropyl-CH₂ ″ ″ 136 61 cyclobutyl-CH₂ ″ ″ 156 62 cyclopentyl-CH₂ ″ ″ 147 63 i-C₄H₉ 2-ClC₆H₄ ″ 182 64 neopentyl ″ ″ 216 65 cyclopropyl ″ ″ 198 66 i-C₄H₉ 4-BrC₆H₄ ″ 135 67 neopentyl ″ ″ 204 68 cyclopropyl ″ ″ 208 69 cyclopropyl-CH₂ ″ ″ 140 70 cyclopentyl-CH₂ ″ ″ 187 71 2-norbornyl-CH₂ ″ ″ 174 72 i-C₄H₉ 3-BrC₆H₄ ″ 152 73 t-C₄H₉ ″ ″ 160 74 neopentyl ″ ″ 177 75 cyclopropyl ″ ″ 186 76 cyclopentyl-CH₂ ″ ″ 143 77 i-C₄H₉ 3,4-diClC₆H₃ ″ 143 78 neopentyl ″ ″ 215 79 i-C₄H₉ 3-CH₃C₆H₄ ″ 119 80 cyclopropyl ″ ″ 206 81 i-C₄H₉ 2-CH₃C₆H₄ ″ 147 82 neopentyl ″ ″ 191 83 cyclopropyl ″ ″ 200 84 i-C₄H₉ 3,4-diCH₃C₆H₃ ″ 165 85 neopentyl ″ ″ 184 86 cyclopropyl ″ ″ 182 87 cyclohexyl ″ ″ 211 88 cyclopentyl-CH₂ ″ ″ 144 89 i-C₄H₉ 3-CF₃C₆H₄ ″ 139 90 cyclopropyl ″ ″ 172 91 cyclopentyl-CH₂ ″ ″ 141 92 i-C₄H₉ 4-CH₃OC₆H₄ ″ 177 93 cyclopropyl ″ ″ 164 94 i-C₄H₉ 3-CH₃OC₆H₄ ″ 119 95 neopentyl ″ ″ 155 96 cyclopropyl ″ ″ 192 97 i-C₄H₉ 2-CH₃OC₆H₄ ″ 181 98 cyclopropyl ″ ″ 211 99 ″ 3,4-diCH₃OC₆H₃ ″ 177 100 i-C₄H₉

″ 158 101 t-C₄H₉

″ 251 102 neopentyl

″ 208 103 cyclopropyl

″ 208 104 i-C₄H₉

″ 193 105 t-C₄H₉

″ 210 106 neopentyl

″ 219 107 cyclopropyl

″ 162 108 i-C₃H₇ 3-NO₂C₆H₄ ″ 176 109 i-C₄H₉ ″ ″ 178 110 neopentyl ″ ″ 229 111 cyclopropyl ″ ″ 234 112 cyclopropyl-CH₂ ″ ″ 164 113 cyclobutyl-CH₂ ″ ″ 150 114 cyclopentyl-CH₂ ″ ″ 183 115 cyclopropyl 3-(CH₃)₂NC₆H₄ ″ 213 116 i-C₄H₉ 2-naphthyl ″ 140 117 cyclopropyl ″ ″ 212 118 i-C₄H₉ 2-thienyl ″ 196 119 cyclopropyl ″ ″ 214 120 i-C₄H₉ 3-thienyl ″ 166 121 cyclopropyl ″ ″ 183 122 i-C₄H₉ C₆H₅ 8-H₃C 170 123 neopentyl ″ ″ 221 124 cyclopropyl ″ ″ 185 125 cyclopentyl-CH₂ ″ ″ 163 126 2-norbornyl-CH₂ ″ ″ 193 127 i-C₄H₉ ″ 9-Cl 174 128 cyclopropyl ″ ″ 149 129 cyclopropyl-CH₂ ″ ″ 175 130 cyclopentyl-CH₂ ″ ″ 175

[0061] The following Examples illustrate pharmaceutical compositions according to the invention.

EXAMPLE 2

[0062] 3,000 inhalation-flasks each containing 40 mg of 3-t-butyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine (active compound) were prepared as follows: Active compound 120 g Sorbitan trioleate 4 g propellent q.s. 60 l

[0063] Procedure

[0064] The microcrystalline suspension prepared with these ingredients was introduced in the inhalation-flasks at a volume of 20 ml per flask with a filling machine. The flasks were furnished with an appropriate valve which released 0.2 ml of suspension for each activation (0.4 mg of active compound).

EXAMPLE 3

[0065] 15,000 capsules each containing 20 mg of 3-t-butyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine (active compound) were prepared from the following formulation: Active compound 300 g Sodium carboxymethyl starch 330 g Talc 195 g Hydrogenated castor oil 165 g Corn starch 495 g

[0066] Procedure

[0067] The above ingredients were sieved through a 60 mesh sieve, then mixed in a suitable mixer and filled into 15,000 gelatine capsules. 

1. A compound of formula (I)

wherein; R¹ represents a hydrogen atom or a —(CH₂)_(m)—Y group, wherein m is an integer from 0 to 4 and Y represents an alkyl, haloalkyl, alkoxy, alkoxycarbonyl, C₃-C₇ cycloalkyl, norbornyl or phenylalkenyl group, or an aromatic group which aromatic group Y may optionally be substituted by one or more halogen atoms; R² represents an aromatic group which aromatic group may optionally be substituted by one or more halogen atoms or alkyl, alkoxy, C₃-C₆ cycloalkoxy, methylenedioxy, nitro, dialkylamino or trifluoromethyl groups; and R³ represents a hydrogen or halogen atom or an alkyl group, and pharmaceutically acceptable salts thereof.
 2. A compound according to claim 1 wherein the alkyl, haloalkyl and alkoxy groups have up to 6 carbon atoms, the alkoxycarbonyl groups have up to 7 carbon atoms and the phenylalkenyl groups have up to 12 carbon atoms.
 3. A compound according to claim 1 or 2 wherein R¹ represents —(CH₂)_(m)—Y wherein m is 0 or 1 and Y represents C₁₋₆ alkyl or C₃₋₇ cycloalkyl.
 4. A compound according to any one of the preceding claims wherein R² represents a phenyl group, naphthyl group or thienyl group which group R² may optionally be substituted by one or more halogen atoms, methyl groups, methoxy groups, cyclopentoxy groups, nitro groups or dimethyl amino groups.
 5. A compound according to claim 4 wherein R² represents a phenyl, 3-chlorophenyl, 4-chlorophenyl, 3-fluorophenyl, 4-fluorophenyl or 3-nitrophenyl group.
 6. A compound according to any one of the preceding claims wherein R³ represents a hydrogen atom, a C₁₋₆ alkyl group or a chlorine atom at the 8- or 9-position of the 1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine skeleton.
 7. A compound according to claim 1 which is 6-(4-fluorophenyl)-3-isobutyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine, 3-cyclopropylmethyl-6-(3-nitrophenyl)-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine, 3-cyclopropyl-6-phenyl-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine and 3-cyclobutylmethyl-6-(3-nitrophenyl)-1,2,4-triazolo[4,3-b]pyrido[3,2-d]pyridazine.
 8. A process for preparing a compound of formula (I)

wherein; R¹ represents a hydrogen atom or a —(CH₂)_(m)—Y group, wherein m is an integer from 0 to 4 and Y represents an alkyl, haloalkyl, alkoxy, alkoxycarbonyl, C₃-C₇ cycloalkyl, norbornyl or phenylalkenyl group, or an aromatic group which aromatic group Y may optionally be substituted by one or more halogen atoms; R² represents an aromatic group which aromatic group may optionally be substituted by one or more halogen atoms or alkyl, alkoxy, C₃-C₆ cycloalkoxy, methylenedioxy, nitro, dialkylamino or trifluoromethyl groups; and R³ represents a hydrogen or halogen atom or an alkyl group, which process comprises formation of the 1,2,4-triazine ring present in formula (I) by cyclisation of a hydrazide of formula (IV)

wherein R¹, R² and R³ are as defined above.
 9. A composition comprising a compound according to any one of claims 1 to 7 or pharmaceutically acceptable salt thereof mixed with a pharmaceutically acceptable diluent or carrier.
 10. A compound according to anv one of claims 1 to 7 or pharmaceutically acceptable salt thereof or a composition according to claim 9 for use in a method of treatment of the human or animal body.
 11. Use of a compound according to any one of claims 1 to 7 or pharmaceutically acceptable salt thereof or a composition according to claim 9 for the manufacture of a medicament for the treatment of a condition whose known treatment is to inhibit phosphodiesterase 4 including allergic reaction and disease states, inflammation, ulcers and immunological disease.
 12. A method of treating a condition whose known treatment is to inhibit phosphodiesterase 4 which comprises administering to a human or animal subject in need of such treatment an effective amount of compound according to any one of claims 1 to 7 or pharmaceutically acceptable salt thereof or a composition according to claim
 9. 